A system for finding or tracking the direction of a particular wave source has many applications. One example is a directional microphone system, where a microphone is to be pointed to the direction of a particular sound source. Another is a video conferencing system, where a camera needs to be moved to the direction of the participating speaker.
One well-known technique of wave-source direction finding is beamforming. Beamforming, itself well-known in the art, uses an array of sensors located at different points in space. Connected to the array of sensors is a spatial filter that combines the signals received from the sensors in a particular way so as to either enhance or suppress signals coming from certain directions relative to signals from other directions.
Where the sensors are microphones, unless two microphones are located at equidistant from a sound source (i.e., arranged so that the line connecting the two microphones is perpendicular to the direction of the sound source), sound originating from the sound source arrives at any two microphones at different times, thereby producing a phase difference in the received signals.
If the received signals are appropriately delayed and combined by changing the spatial filter coefficients, the behavior of the microphone array can be adjusted such that it exhibits maximum receiving sensitivity toward a particular direction. In other words, the direction of maximum receiving sensitivity (so called "looking direction" of the microphone array) can be steered without physically changing the direction of the microphone array. It is then possible to determine the direction of a particular sound source by logically (computationally) steering the looking direction of the microphone array over all directional angles and looking for the angle that produces the maximum signal strength.
However, the use of beamforming for sound-source direction finding has several drawbacks. First, a typical beamforming profile of receiving sensitivity over the angles of looking direction is so flat that it is, as a practical matter, difficult to find the peak point of maximum signal strength unless an inconvenience of a large microphone array is used. For example, the 3-dB attenuation points (reference points for signal discrimination) of a typical 15-cm microphone array may be separated by as much as 100 degrees. At small angles such as 5 degrees, the corresponding attenuation is insignificant. As a result, even a slight numerical error or noise may perturb the result, giving an erroneous direction.
Second, beamforming involves scanning the space for the direction producing the maximum received signal strength. Finding the source direction in terms of a horizonal direction (azimuth) and a vertical direction (elevation) involves searching two-dimensional space, which is computationally expensive.
Third, in order to determine the source direction with a high spatial accuracy, it is necessary to perform the beamforming calculation at a very high resolution (for example, every 1 degree). This requires delaying and summing the received signals at a very small delay step, which, in turn, requires that the signal be sampled at a very high sampling rate, imposing a severe computational burden.
Another method of finding the direction of a sound source is to measure time delays between a pair of sensors. For example, Hong Wang & Peter Chu, Voice Source Localization for Automatic Camera Pointing System in Videoconferencing, Proc. IEEE International Conference on Acoustics, Speech, and Signal Processing, April 1997, pp. 187-90, disclose an array of microphones mounted on a vertical plane, three of them arranged in a horizonal line, and the fourth located above the center one of the three. The horizontal direction of a sound source (azimuth) is calculated by measuring the time delays of incoming signals between the two remote microphones in the horizontal line. The vertical direction (elevation) is calculated by measuring the time delays of incoming signals between the center microphone in the horizontal line and the upper microphone.
The Wang & Chu system has several drawbacks. First, since all the microphones are on the same vertical plane, they produce the same delay whether sound is coming from the front or from the back. Since the system cannot distinguish between a front and a back, ambiguities are inevitable.
Second, the performance of the system is not symmetric with respect to looking sideways and looking forward. The capability of such a system to resolve and estimate the direction of a source depends on the change of time delay in response to an incremental change in the angular direction. The time delay between two incoming signals at two adjacent microphones is: EQU time delay=sin (.phi.)*aperture/sound-velocity,
where .phi. is the angle of arrival of the sound waves, measured with respect to the normal of the microphone array, and the aperture is the spacing between two nearest microphones. Note that the change in time delay is obtained as the derivative of sin(.phi.), which is a function of cos(.phi.). For the same incremental angular change, the resulting time delay when the looking direction is sideways (when .phi. approaches 90 degrees) is smaller than when the looking direction is forward (when .phi. approaches 0 degree). As a result, the performance of the system looking sideways is poorer than that looking forward.
Third, the Wang & Chu system does not provide any indication of how reliable the measurements used for the direction determination were. The time delay measurement between a pair of microphones may not be reliable if it was measured in the presence of noise or based on non-relevant signals. The quality of measurement would be poor if the measurement were made in a noisy environment. Also, even if the measurement were of high quality, it may not be relevant to the direction determination. For example, if the time delay measurement were a measurement of reflected sound from a wall or furniture, or sound from a repeater source such as a loud speaker connected to an audio/video conferencing system, the measurement may not even be relevant to the direction determination. The Wang & Chu system does not provide any mechanism for verifying the quality or relevancy of measurement.
Therefore, there exists a need for a system and method that can determine the direction of a wave source accurately and efficiently, and that can also indicate the quality and relevancy of the measurements on which the direction determination is based.